1. Field of the Invention
The present invention relates to a resonant element used as an angular velocity sensor, filter, or the like.
2. Description of the Related Art
FIG. 7A is a perspective view showing a previous resonant element 16. The resonant element 16 is a microelement produced utilizing a conventional silicon micromachining technique and the like. More specifically, the resonant element 16 is produced by forming a nitride film 7 on a silicon substrate 1, then forming a polysilicon film 5 thereover, and forming the films 7 and 5 into a predetermined pattern by dry etching or the like.
The substrate 1 functions as a fixed substrate of which the substrate plane direction is an X-Z two-dimensional plane direction. A weight portion 2 is disposed above the substrate in a state isolated from the substrate 1. In the resonant element 16 shown in FIG. 7A, the weight portion functions as a planar vibrating body 10. The planar vibrating body 10 is supported via support beams 3 so as to be vibratable in the X-direction. One end side of each of the support beams 3 is fixed to the substrate I via a fixing portion 35.
Comb electrodes 6B are formed on both sides of the planar vibrating body 10 outwardly in the transverse direction (X-direction), and comb electrodes 6A are each disposed inwardly in the transverse direction at positions opposed to and interdigitated with the comb electrodes 6B. Conductive layers for driving 11A and 11B are connected to the comb electrodes 6A and 6B, respectively, and are connected with outside electrode pads (not shown) via conductor patterns (not shown), and thus form an exciter 4.
Once an AC voltage is applied to these conductive layers for driving 11A and 11B of the exciter 4, an electrostatic force is generated between the comb electrodes 6A and 6B, and the planar vibrating body 10 is vibrated in the arrow F direction (X-direction) by this electrostatic force.
When the resonant element 16 is rotated around the Z-axis while the planar vibrating body 10 is vibrated in the X-direction by driving the comb electrodes 6A and 6B, a Coriolis force occurs in the Y direction orthogonal to the above-described X-Z two-dimensional plane direction. The Coriolis force is applied to the planar vibrating body 10 constituted of the weight portion 2, and the planar vibrating body 10 vibrates in the direction of the Coriolis force. By measuring an electric signal corresponding to the magnitude of the vibration amplitude of the planar vibrating body 10 due to the Coriolis force, for example, the magnitude of the rotational angular velocity can be detected.
In the case where the resonant element 16 is used as an angular velocity sensor, there is provided a detecting portion for measuring the electric signal corresponding to the magnitude of the vibration amplitude of the planar vibrating body 10 due to the Coriolis force.
When the resonant element 16 is produced, the resonance frequency of the planar vibrating body 10 in the direction of the Coriolis force (Y-direction) is previously set at the design stage to the resonance frequency in the X-direction, and the shape, dimension, weight, etc. of the planar vibrating body 10 are designed and produced so that the above-mentioned resonance frequency is obtained. In many cases, however, the shape, dimension, weight, etc. of the planar vibrating body 10 are not achieved as designed, because of the machining accuracy of silicon micromachining technique. Accordingly, deviation of the resonance frequency of the planar vibrating body 10 from the designed frequency often occurs. If the vibration of the planar vibrating body 10 is in a resonant state, the amplitude is greatly amplified by virtue of the value of the Q (quality factor) related to the structure, but if the frequency deviates, a problem arises in that the amplitude is not nearly amplified as much, resulting in the sensitivity of the resonant element begin significantly reduced. It is, therefore, necessary to perform trimming with respect to the weight portion 2 and/or the support beams 3 by, for example, a complicated machining process, to thereby adjust the resonance frequency of the planar vibrating body 10 to the design frequency.
Since the resonant element 16 is, however, a minute resonant element 16, it is practically impossible because of the accuracy of conventional mechanical trimming techniques to perform trimming of the minute weight portion 2 and/or support beams 3 so as to have the desired dimensions, shape, and weight, etc. It is, therefore, very difficult to adjust the resonance frequency of the planar vibrating body 10 to a set value.
Therefore, in the resonant element 16, as shown in FIG. 7B, a conductive layer 12 for providing an electrostatic attractive force 15 is located at a position opposed to the weight portion 2 in the Y-direction with a gap therebetween. As shown in FIG. 7A, the conductive layer 12 is connected to a conductive pad 14 via a conductive pattern 13. By controlling the voltage to be applied to the conductive layer 12 via the conductive pattern 13 and conductive pad 14, the resonance frequency of the resonant element 16 is adjustable to a set value.
Once a DC voltage is applied to the conductive layer 12, an electrostatic force acts on the planar vibrating body 10 as an electrostatic spring. Specifically, when the planar vibrating body 10 vibrates in the direction such that the planar vibrating body 10 approaches the substrate 1, an electrostatic force acts in the direction such that the amplitude is increased, and hence the application of the DC voltage to the conductive layer 12 has an effect of generating a force in the opposite direction as if a mechanical spring were being compressed. This results in a reduction in the resonance frequency in the Y-direction. Since this reduced amount of the resonance frequency varies in accordance with the electrostatic attractive force 15, a fine-adjustment of the resonance frequency of the planar vibrating body 10 from the natural frequency thereof to the lower frequency side can be performed by adjusting the magnitude of the DC voltage applied to the conductive layer 12.
Utilizing this effect, by designing the natural resonance frequency of the planar vibrating body 10 in the Y-direction to be slightly higher than the most sensitive resonance frequency (the resonance frequency in the X-direction), i.e., by designing the resonance frequency of the planar vibrating body 10 in the detection direction to be higher than the resonance frequency thereof by the exciter 4 in the vibrational direction, the sensitivity of the resonant element 16 can be increased by adjusting the DC voltage applied to the conductive layer 12.
In the resonant element 16, it is important to adjust the resonance frequency thereof to a set value and to keep the vibrating state of the planar vibrating body 10 on-target. FIGS. 6A and 6B illustrate examples of movements of a planar vibrating body 10 in the X-Y plane without angular velocity around the Z-axis, when the planar vibrating body 10 is vibrated in the X-direction. In the resonant element 16 shown in FIGS. 7A and 7B, if the vibration of the planar vibrating body 10 is deflecting in the Y-direction, which is the detection direction, that is, if the planar vibrating body 10 tilts with respect to the substrate plane, a Coriolis force cannot be accurately measured if this tilt is substantial, and the gyro characteristics of the angular velocity sensor or the like deteriorates.
It is therefore desirable that the vibratory state of the planar vibrating body 10 hardly exhibits any deflection in the Y-direction, as shown in FIG. 6B.
Generally, the less the difference (xcex94f) in the resonance frequency of the planar vibrating body 10 between the vibrational direction and detection direction, the larger the mechanical coupling between the two directions (the propagation of a mechanical energy and the interaction between the two vibration modes) becomes and the larger the deflection in the detection direction while the resonant element 16 is driven becomes. In particular, the dimensional error or the residual stress when the resonant element is produced, increases this mechanical coupling.
In the resonant element 16, therefore, even if the difference (xcex94f) in the resonance frequency of the planar vibrating body 10 between the vibrational direction and detection direction is reduced in order to increase the sensitivity thereof, a Coriolis force can not be accurately measured, if the deflection amount in the detection direction increases. As a result, a resonant element 16 having high sensitivity and accuracy can not be obtained only by reducing the above-described difference (xcex94f) in the resonance frequency through providing a conductive layer 12. It has therefore been difficult to achieve a resonant element 16 wherein the difference (xcex94f) in the resonance frequency is small and wherein the deflection in the detection direction is small, and the yield of the resonant elements 16 capable of meeting both characteristics has been very low.
In principle, it is possible to perform mechanical trimming in a conventional resonant element 16 having a dimensional error or the like so as to reduce the deflection amount of the planar vibrating body 10 in the detection direction; however, from a practical standpoint, it is not practicable to perform mechanical trimming while evaluating the deflection amount of the planar vibrating body 10.
It is also impractical and would take an extremely long time to bring the deflection amount to zero by performing repeated trimming operation in such away that the deflection amount of the planar vibrating body 10 is ascertained after trimming, and that trimming is again performed. Accordingly, there is a need for a resonant element 16 which allows the difference (xcex94f) in the resonance frequency of the planar vibrating body 10 between the vibrational direction and the detection direction to be small and which allows the deflection in the detection direction to be small, without the need for the above-described repetitive trimming.
The present invention has been made in order to solve the above-described problems. It is an object of the present invention to provide a resonant element allowing both the difference (xcex94f) in the resonance frequency of the planar vibrating body 10 between the vibrational direction and the detection direction and the deflection in the detection direction to be small, without the need for troublesome trimming.
In order to achieve the above-described object, the present invention has the following constitutions. In a first aspect, a weight portion is disposed above a fixed substrate in a state isolated from said fixed substrate, of which the substrate plane direction is an X-Z two-dimensional plane direction; a planar vibrating body comprising said weight portion is supported by said fixed substrate via support beams so as to be vibratable in an X-direction; an exciter for vibrating the planar vibrating body in the X-direction is provided; and vibrating body tilt correcting means for adjusting the resonance frequency of the planar vibrating body by giving electrostatic forces to the planar vibrating body thereby correcting the tilt of the planar vibrating body with respect to the substrate plane direction of the fixed substrate.
In accordance with another aspect, the tilt correcting means are provided at least at the both edge areas of the planar vibrating body with a gap therebetween in the X-direction and are spaced from the vibrating body in a Y-direction orthogonal to the X-Z two-dimensional plane direction.
In accordance with another aspect, the planar vibrating body comprises a frame body disposed above the fixed substrate in a state isolated from the fixed substrate, and a weight portion connected to the inside of the frame body by connection beams. First vibrating body tilt correcting means are provided at least at the both edge areas of the weight portion with a gap therebetween in the X-direction and spaced from the vibrating body in the Y-direction. Second vibrating body tilt correcting means are provided at positions opposed to the frame body and across the first vibrating body tilt correcting means via gaps in the X-direction.
In accordance with another aspect, the stress canceling means are provided which directly or indirectly applies to the support beams a force in a direction such that the tensile stresses within the support beams, are canceled, the tensile stresses being caused by electrostatic attractive forces given to the planar vibrating body by the vibrating body tilt correcting means.
In accordance with another aspect, the stress canceling means are arranged so as to sandwich the planar vibrating body between the stress canceling means and the vibrating body tilt correcting means via gaps.
In accordance with another aspect, a vertical movement side electrode is provided on at least one of the front surface and rear surface of the weight portion, and a fixed opposing electrode is provided on the side opposed to the vertical movement side electrode with a gap interposed in the Y-direction; and the set of the vertical movement side electrode and the fixed opposing electrode are formed as a detecting electrode for detecting the vibration amplitude of the weight portion in the Y-direction caused by a variation in an angular velocity of rotation applied to the resonant element around the Z-axis.
In accordance with another aspect, the weight portion is formed of silicon or polysilicon, and constitutes a vertical movement side electrode in itself.
In the present specification and claims, the term xe2x80x9cboth edge areasxe2x80x9d represents a wider concept including areas somewhat inside both edge portions or areas somewhat outside both edge portions in the planar vibrating portion or the weight portion.
In accordance with the present invention, both the adjustment of the resonance frequency of the planar vibrating body and the correction of the tilt of the planar vibrating body with respect of the substrate plane direction of the fixed substrate can be performed by the described vibrating body tilt correcting means. It is thereby possible, without the need for troublesome trimming, to reduce the difference between the vibrational direction of the planar vibrating body vibrating by a Coriolis force and the detection direction thereof, as well as to reduce the deflections in the detection direction, and to create thereby a superior resonant element having a high sensitivity and a low noise level.
In the resonant element wherein the planar vibrating body comprises a frame body and a weight portion, and wherein first vibrating body tilt correcting means are provided at least at both edge areas of the weight portion, and wherein second vibrating body tilt correcting means are provided at positions opposed to the frame body and across the first vibrating body tilt correcting means via a gap in the X-direction, the resonance frequency of the planar vibrating body comprising the weight portion and the frame body can be adjusted, and the tilt of the weight portion and the frame body with respect to the plane of the substrate can be individually corrected.
Furthermore, in the resonant element in accordance with the present invention, when the weight portion is connected to the inside of the frame body by the connection beams, the movement of the frame body and the weight portion can be made independent of each other by the construction of the connection beam so that, for example, when the planar vibrating body vibrates in the X-direction and rotates around the Z-axis, only the weight portion vibrates in the Y-direction but the frame body hardly vibrates. This allows the planar vibrating body to perform excitation vibration more stably in the vibrational direction.
Moreover, in the resonant element in accordance with the present invention wherein stress canceling means is provided, since a force in a direction such that tensile stresses within the support beams caused by electrostatic forces given to the planar vibrating body by the vibrating body tilt correcting means are counteracted can be applied to the support beams directly or indirectly by the stress canceling means, the occurrence of various problems caused by any troublesome tensile stresses within the support beams can be reliably avoided. This allows a resonant element having a higher sensitivity and a lower noises to be provided.
Also, in accordance with the present invention wherein a vertical movement side electrode is provided on at least one of the front surface and rear surface of the weight portion (or where the weight portion itself serves as the vertical movement side electrode), and wherein a fixed opposing electrode is provided at the side opposed to the vertical movement side electrode with a gap interposed in the Y-direction, and wherein the set of the vertical movement side electrode and the fixed opposing electrode are constituted as a detecting electrode for detecting the vibration amplitude of the weight portion in the Y-direction due to an angular velocity around the Z-axis, a variation in an angular velocity of the rotation around the Z-axis can be accurately detected by the detecting electrode.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.